In many demanding application, such as components for gas turbines, rocket engines and other types of structural members for which a high reliability is required, a brittle fracture behavior is unacceptable. Brittle materials often lead to fracture without plastic deformation having preceded the fracture, or if the breaking stress is locally exceeded following the stress concentration which arises near a microcrack or other defect. This reduces the possibility of detecting faulty parts by inspection or estimating service lives by statistical methods and replacing these parts prior to a breakdown.
The above applies preferably to ceramics which are very strong but for which the strength is reduced and the variation in strength values between individual parts becomes great as a consequences of the inability of absorbing stresses by plastic deformation as well as the occurrence of internal defects in the form of microcracks and other inhomogeneities. Thus, they result in fracture when the weakest link breaks. The same is also true of certain metallic materials, preferably in high-temperature applications, where changes in the microstructure as a result of diffusion and grain growth increase the tendency to brittle fracture behavior. One way of avoiding brittle fractures is to reinforce with fibers. Fiber reinforcement gives greater fracture toughness and safety against fracture because when the strength is locally exceeded, the fracture does not propagate spontaneously through the part but the fiber take up and redistribute the stresses in the same way as when one of a large number of parallel-connected links breaks. In this way, defects can be detected and critical components be replaced during a regular overhaul.
Known methods for manufacturing ceramics reinforced with long fibers are of two main types, namely:
Preform infiltration: A three-dimensional fiber preform is manufactured. The preform is then infiltrated with a ceramic matrix. During the infiltration the ceramic matrix may be supplied:
in gaseous phase, whereby the fibers included in the fiber preform constitute a substrate on which the ceramic matrix is built by precipitation from the gaseous phase, Chemical Vapor Infiltration (CVI); PA1 in liquid form, whereby the ceramic particles which build up the ceramic matrix are supplied to the fiber preform suspended in a liquid polymer, in the form of a sol or suspended in a powder slurry, and the built-up ceramic matrix is bonded to the fiber preform by reaction with a gas. Irrespective of how the fiber preform is infiltrated, the ceramic matrix is built in the void between the fibers without the fibers being packed more densely. Ceramic fiber composites produced by infiltration in the manner described above typically result in a material with a porosity of from 10 to 15 per cent by volume.
Uniaxial pressure sintering: Fibers are mixed with ceramic powder and are formed into a green body. The green body is consolidated and sintered under uniaxial pressure. Often the ceramic matrix comprises a ceramic with a low melting point which is densified via a viscous flow. Typical matrices for ceramic fiber composites produced by means of uniaxial pressure sintering are glass and glass ceramics in the X-aluminium silicate system where X preferably consists of any of the substances Li, Mg, Ca, Zr or Y. During the pressure sintering both the ceramic matrix and the fibers are packed together, and completely dense materials can be obtained under favorable conditions. Also green bodies formed by winding of infiltrated fiber bundles have been consolidated and sintered by means of uniaxial pressure sintering. The applicability of the method, because the pressure is applied uniaxially, is limited to simple geometries. Also limitations as regards magnitude and compacting pressure are very pronounced in the case of uniaxial pressure sintering.
In addition to the methods described above, test bars, with simple geometries of silicon nitride containing continuous carbon fibers have been manufactured by enclosing green bodies in glass, consolidating the bodies and sintering them by means of hot-isostatic pressing.